Method for forming shallow trench isolations

Abstract

A method for forming shallow trench isolations includes the steps of defining a wafer substrate, forming a silicon dioxide insulating layer on the substrate, depositing a silicon nitride layer on the silicon dioxide insulating layer, and forming at least one trench in the substrate through the silicon dioxide and silicon nitride layers. The method also includes the steps of depositing a silicon dioxide layer over the silicon nitride layer and in the trench, removing the silicon dioxide layer deposited over the silicon nitride layer, anisotropically etching the silicon dioxide layer to produce silicon dioxide sidewalls in the trench contiguous with the silicon nitride layer, isotropically etching to remove the sidewalls and removing the silicon nitride layer.

What is claimed is:  
     
       1. A method for forming planar shallow trench isolations in a wafer substrate having a silicon dioxide insulating layer disposed over the substrate and a silicon nitride layer disposed over the silicon dioxide insulating layer to form a substrate-insulator-silicon nitride stack, a surface of the substrate-insulator-silicon nitride stack being planar and having a plurality of trenches filled with silicon dioxide, comprising the steps of: 
       anisotropically etching the silicon dioxide layer to produce in the trenches silicon dioxide having sidewalls contiguous with the silicon nitride layer; and  
       isotropically etching to remove said silicon dioxide sidewalls such that the silicon dioxide in the trenches is planar with the silicon dioxide insulating layer.  
     
     
       2. The method as claimed in  claim 1  wherein said step of isotropically etching comprises a step of performing a buffered oxide etch. 
     
     
       3. The method as claimed in  claim 1  wherein said step of anisotropically etching comprises a step of etching with faster etch rate at the edge of the wafer.